We assess the impact of air masses transported from the Po basin on the particulate matter (PM) levels in the northwestern Alps using multi-sensor observations from ground and space, and models. In this part 1 of the study, we investigate the phenomenon through three selected case studies representative of different seasons. We show that advected aerosols remarkably degrade the air quality of the Alpine area (PM₁₀ increasing up to >100 µg m⁻³) and we discuss the measurement–model discrepancies.